This invention relates to gate valves and, in particular, to gate valves lined with wear-resistant or corrosion-resistant material.
Gate valves are often used to control the flow of abrasive slurries transported through tubular conduits. Gate valves used for this purpose are often lined with resilient material such as polyurethane. The resilient lining protects the valve from rapid wear caused by the abrasive solids in the slurry.
Gate valves generally include a body having a lower portion that is secured between the ends of two axially aligned conduits. The upper portion of the body extends upwardly from the conduits. The lower portion has a port formed through it. The shape of the port is usually circular, substantially conforming to the shape of the opening in the conduits. The port provides a fluid passage between the two conduits. The body also has a slot formed in it. The slot is configured to receive a thin flat gate that slides within the slot. The slot is configured so that the gate can slide across the port in the lower portion of the body. To close the gate valve, the gate is extended far enough into the lower portion of the valve body to occlude the fluid passage between the two conduits. The upper portion of the valve body carries suitable mechanisms (threaded stem, handwheel, etc.) for controlling the movement of the gate.
It is important that the valve gate seat properly when closed. That is, to prevent leakage, an effective seal must be formed between the closed gate and the part of the body that defines the port. One common method of ensuring that the gate is properly seated utilizes the upstream pressure in the fluid carried by the conduits to force the gate against a seal ring that is located around the valve port on the downstream side of the gate. The pressure of the fluid exerted against the closed gate promotes an effective seal between the gate and the seal ring. Typically, two or more sealing wedges are mounted to the lower portion of the body at spaced-apart locations around the port just upstream of the gate. These sealing wedges guide the gate against the seal ring as the gate is moved into the closed position. In some lined gate valves, such as described in U.S. Pat. No. 4,201,365 issued to Paptzun et al., the seal ring is a replaceable unit formed of resilient material.
Valves that utilize the fluid pressure in the upstream conduit to promote the sealing of the gate often leak when the pressure in the conduit drops. The leakage is a result of the drop in fluid pressure, which reduces the sealing force of the gate against the seal ring. Gate valves can be designed to prevent leakage at low pressure by configuring the valve to provide a tight fit between the gate and seal ring at low pressure; however, a gate valve so configured can not be used with conduit systems in which fluid pressure can vary over a wide range of amounts because as the amount of fluid pressure in the conduit increases, the gate will seat so tightly against the seal ring that it becomes very difficult to operate the valve.